1. Field of the Invention
The field of the invention is that of remote image acquisition, and in particular the acquisition of images transmitted terrestrially or by satellite. More precisely, the invention concerns a method for automatically georeferencing the remote acquisition of images, meaning that the invention concerns a method that is used to automatically associate, with the elements for the remote acquisition of images, a position that is determined by geographical or cartographic coordinates.
2. Description of the Prior Art
A remotely-acquired image supplies a three-dimensional, spatially-sampled representation of a region on the ground called a scene. This is a representation in which each digital remotely-acquired image is representative of a small area on the ground, centred on each of the points of the sampling grid.
The majority of remotely-acquired imagery applications necessitate a method called georeferencing, which establishes a geometrical relationship between the coordinates in the image geometry, defined by a line number and a column number, and the geographical or cartographic coordinates on the ground.
The correspondence between the line and column image coordinates and the ground coordinates, which are expressed in terms of latitude and longitude in the geographical coordinate system and in meters in the projection cartographic system, is determined by calculating the intersection of the direction of view of the detectors making up the sensor of the imaging system that acquires the images, at the moment of acquisition, using a model of the Earth.
This calculation then requires two categories of information.
The first information category concerns knowledge of the geometry of the picture. This is determined by the characteristics of the sensor (instrument characteristics), the position, the speed and the attitude of the carrier, where the latter are the angles of roll, pitch and yaw. The picture geometry is known to the user through a geometrical model, created by means of geometrical sampling, and delivered in general with the images. This model is always of limited precision, through lack of sampling of the focal plane, and because of sampling and filtering of the attitude data.
The second information category concerns knowledge of an altitude model of the Earth. The latter can be approximated by an ellipsoid or by an altitude model of the region in which the scene is located, and more particularly a digital terrain model (DTM). This is defined as a digital representation of altitude that includes the gradient and relief parameters of the geographical region under study.
Being in possession of such information with any degree of precision is rather unusual in practice. The raw images obtained by a sensor cannot be georeferenced directly in a precise manner, since they include global or local deformations that are not modelled perfectly. One could mention, for example, the deformations due to imperfect knowledge of the picture optics, to unknown movements of the imaging system, and to inaccuracies in one's knowledge of the relief in the observed zone.
When it is desired to geometrically correct and georeference a remotely-acquired image, it is then necessary to call upon additional information that can chiefly be either knowledge of a certain number of points called control points, or knowledge of other remote image acquisitions that have already been georeferenced.
The first known technique employed to correct and georeference a remotely-acquired image is therefore dependent upon the search for control points, that is for known locating points in terms of coordinates, and which are identifiable both in the image and on the ground. With knowledge of the local relief (from the DTM) where available, these points are used to refine the geometrical picture model of the remotely-acquired image, which then allows its projection on the ground and enables it to be georeferenced.
The projection on the ground increases in precision with the number of control points available and with the evenness of their distribution in the image.
The control point selection procedure generally makes use of points that are well-defined and precise, such as road intersections, buildings, corners of fields, etc.
In most cases, selection is effected from a map. When no map is available, or its precision is inadequate, then one has to resort to finding control points by conventional geodesic methods or by use of the global positioning system (GPS).
The selection of control points is thus partially or totally manual. This is a lengthy and costly operation that calls for very experienced operators and a great deal of care in order to avoid the introduction of errors into the georeferencing process.
The second known, technique to correct and georeference a remotely-acquired image, and which is used in order to circumvent the manual process, relies on the use of a certain number of thumbnail images, which are extracted from images that have already been georeferenced.
In this case, the search for control points is replaced by the search for validated points between the image to be georeferenced and the already georeferenced thumbnail image, which has the advantage of allowing the georeferencing procedure to be automated.
Now this technique assumes that one is in possession of a database of images or of thumbnail images that cover a large part of the surface of the Earth, and that this base is available for most of the users of image processing. At the present time, such a database, if any, is not freely accessible.
In addition, the constitution of such a database is a complex and extremely expensive operation.
Moreover, if use is made of such a reference base, then the large seasonal variations of the images give rise to a lack of temporal correlation that complicates the search for validated points between the image to be georeferenced and the reference image.
One object of this present invention is to propose an image processing system that achieves the georeferencing of images and that also differs from the previous techniques and their drawbacks, in particular in that it allows a fully automatic georeferencing of images that does not depend upon on a database of images or on thumbnail images that have already been georeferenced.
Another object of the invention is to provide a method for the georeferencing of images that is entirely automatic, from an image pair acquired either by CCD (charge-coupled device) sensors, or by TDI (time delay integration) matrices, with resolutions and spectral bandwidths that may be different, belonging to the same focal plane and therefore arranged in a configuration with a small stereoscopic base.
The stereoscopic coefficient, b/h, for each point on the ground, is the ratio between the distance, b, between the two points in the space that have imaged this point on the ground, and the distance, h, between these points in the space and the point in the observed scene. A small stereoscopic base configuration is defined as a stereoscopic ratio, b/h, that is typically less than 0.3.
Another object of this present invention is to overcome, in the georeferencing process, the image artifacts that are characteristic of stereoscopy at low b/h, namely the residues arising from unmodelled attitude vibrations. Attitude residues can be seen in FIG. 1, and are indicated schematically by waves along the columns, causing the appearance alternately of sets of light and dark lines such as sets a′, b′, and c′. If they were not eliminated, these attitude artifacts would be interpreted as alternately positive and negative variations of the altitude value calculated around the true value.
It is also desirable to offer a technique for the georeferencing of remote image acquisitions that, firstly, allows use of the data available to any user and, secondly, achieves a saving in terms of cost and time while also preserving the quality of georeferencing precision. Apart from this, the precision achieved by the method is better than the location accuracy of most of the imaging systems that are operational currently.